The present invention relates generally to processes for purification of hydrocarbons and, more particularly, to adsorption processes using sorbents selective to sulfur compounds and to aromatic compounds.
Petroleum is an extremely complex mixture and consists predominantly of hydrocarbons, as well as compounds containing nitrogen, oxygen, and sulfur. Most petroleums also contain minor amounts of nickel and vanadium. The chemical and physical properties of petroleum vary considerably because of the variations in composition.
The ultimate analysis (elemental composition) of petroleum tends to vary over relatively narrow limits—carbon: 83.0 to 87.0 percent; hydrogen: 10.0 to 14.0 percent; nitrogen: 0.1 to 1.5 percent; oxygen: 0.1 to 1.5 percent; sulfur: 0.1 to 5.0 percent; metals (nickel plus vanadium): 10 to 500 ppm.
Crude oils are seldom used as fuel because they are more valuable when refined to petroleum products. Distillation separates the crude oil into fractions equivalent in boiling range to gasoline, kerosene, gas oil, lubricating oil, and residual. Thermal or catalytic cracking is used to convert kerosene, gas oil, or residual to gasoline, lower-boiling fractions, and a residual coke. Petrochemical intermediates such as ethylene and propylene are primarily produced by the thermal cracking of light hydrocarbon feedstocks in the presence of steam. Catalytic reforming, isomerization, alkylation, polymerization, hydrogenation, and combinations of these catalytic processes are used to upgrade the various refinery intermediates into improved gasoline stocks or distillates. The major finished products are usually blends of a number of stocks, plus additives.
Gasoline is a complex mixture of hydrocarbons that distills within the range 100 to 400° F. Commercial gasolines are blends of straight-run, cracked, reformed, and natural gasolines. Straight-run gasoline is recovered from crude petroleum by distillation and contains a large proportion of normal hydrocarbons of the paraffin series. Cracked gasoline is manufactured by heating crude-petroleum distillation fractions or residues under pressure, or by heating with or without pressure in the presence of a catalyst. Heavier hydrocarbons are broken into smaller molecules, some of which distill in the gasoline range. Reformed gasoline is made by passing gasoline fractions over catalysts in such a manner that low-octane-number hydrocarbons are molecularly rearranged to high-octane-number components. Many of the catalysts use platinum and other metals deposited on a silica and/or alumina support. Natural gasoline is obtained from natural gas by liquefying those constituents which boil in the gasoline range either by compression and cooling or by absorption in oil.
Removal of the sulfur-containing compounds is an important operation in petroleum refining, and is often achieved by catalytic processes at elevated temperatures and pressures. The hydrodesulfurization (HDS) process is efficient in removing thiols and sulfides, but much less effective for heterocyclic diunsaturated sulfur compounds, such as thiophenes and thiophene compounds/derivatives (e.g., benzothiophene and dibenzothiophene).
In 1998, the U.S. largest automakers pledged to put clean-burning cars on the road by 2001, beating the Clean Air Act Amendments mandate by five years. They proposed the use of internal combustion (IC) engines capable of emitting 50% fewer nitrogen oxides (NOx) and 70% fewer hydrocarbons, thanks to advanced catalytic converters. Shortly after this low-emission vehicle concept was announced, the U.S. Environmental Protection Agency (EPA) revealed concerns that these reductions might not be achievable if high-sulfur gasoline and diesel fuel continue to be used. Studies involving the EPA and the automobile and oil industries showed that fuel sulfur atoms can bond with reactive sites on the catalyst surface, preventing catalyzed reactions needed to break down NOx and hydrocarbons. Since high-sulfur gasoline may perhaps decrease the effectiveness of advanced catalytic converters, the EPA mandates a reduction in gasoline and diesel sulfur levels to 30 and 15 ppm, respectively, down from the current levels of 300–500 ppm. This should be attained by the year 2006. Faced with the severely high costs of compliance, a surprising number of refiners are seriously considering reducing or eliminating production of on-board fuels.
Ultra-clean fuel may also be desirable for use with a fuel cell system. For the automotive fuel cells, liquid hydrocarbons may be ideal fuels due to their higher energy density, availability, and safety for transportation and storage. However, liquid hydrocarbons usually contain certain sulfur compounds that are poisonous to both the shift catalysts in the hydrocarbon fuel processors and the electrode catalysts in fuel cell processes. Thus, the sulfur content in the liquid hydrocarbons would desirably be generally less than about 0.1 ppm.
During the last decade, there have been several published accounts on using adsorption for liquid fuel desulfurization. Commercially available sorbents (i.e., zeolites, activated carbon and activated alumina) were used in all of these studies. Weitkamp et al. reported that thiophene adsorbed more selectively than benzene on ZSM-5 zeolite. See Weitkamp, J.; Schwark, M.; Ernest, S. “Removal of Thiophene Impurities from Benzene by Selective Adsorption in Zeolite ZSM-5,” J. Chem. Soc. Chem. Commun. (1991), 1133. Without being bound to any theory, it is believed that this is because thiophene (C4H4S, also known as thiofuran) has a higher dipole moment (0.55 debye) than benzene (non-polar), although their polarizabilities are similar. Based on this study, King et al. studied selective adsorption of thiophene, methyl- and dimethyl-thiophenes (all with one ring) over toluene and p-xylene, also using ZSM-5. See King, D. L.; Faz, C.; Flynn, T. “Desulfurization of Gasoline Feedstocks for Application in Fuel Reforming,” SAE Paper 2000-01-0002, Soc. Automotive Engineers, Detroit (2000). They showed that thiophene was more selectively adsorbed, both based on fixed bed breakthrough experiments. However, the capacities for thiophene were unfortunately quite low (1–2% wt. adsorbed at 1% thiophene concentration). Both vapor phase and liquid phase breakthrough experiments were done in these studies, and the results from two phases were consistent.
The pore dimensions of ZSM-5 are 5.2–5.6 Å. Hence, organic sulfur compounds with more than one ring will be sterically hindered or excluded. Zeolites with larger pores, as well as larger pore volumes, would appear to be more desirable than ZSM-5 as the selective sorbents. Indeed, results of Salem and Hamid indicated that 13X zeolite as well as activated carbon had much higher sorption capacities for sulfur compounds. See Salem, A. S. H.; Hamid, H. S. “Removal of Sulfur Compounds from Naphtha Solutions Using Solid Adsorbents,” Chem. Eng. Tech. (1997), 20, 342. Based on the data of Salem and Hamid, Id., the capacity for sulfur compounds by 13X zeolite was approximately an order of magnitude higher than that of ZSM-5, when compared with the data of King et al. (cited above) extrapolated to the same conditions.
Activated alumina (Alcoa Selexsorb) has been used in an adsorption process.
No direct comparison has been made among these commercial sorbents. Their experiments were mostly done in fixed bed adsorbers, by measuring the breakthrough capacities. Based on the literature, the large pore zeolites (NaX or NaY) are about the same as activated carbon and alumina, in terms of adsorption of thiophene.
As mentioned hereinabove, current sulfur levels found in commercial liquid fuels are commonly obtained by Hydrodesulfurization (HDS) treatment. The HDS method is very effective in removing thiols and sulfides, but it is generally not adequate for the removal of thiophenic compounds. For instance, the H2S produced during reaction of some thiophene derivatives is one of the main inhibitors for deep HDS of unreactive species. Ma, X.; Sakanishi, K.; Mochida, I. Hydrodesulfurization Reactivities of Various Sulfur Compounds in Diesel Fuel. Ind. Eng. Chem. Res. (1994), 33, 218 and Knudsen, K. G.; Cooper, B. H.; Topsøe, H. Catalyst and Process Technologies for Ultra Low Sulfur-Diesel. Appl. Catal. A-Gen. (1999), 189, 205. For HDS to meet the new federal government mandates, it is believed that reactors with volumes 5–15 times larger (depending on the H2 pressure) than those currently used may be needed.
Ma et al. studied fixed-bed adsorption of thiophene compounds from jet fuels and diesel using an undisclosed transition metal compound (5 wt % loading) supported in silica gel. Ma, X.; Sun, L.; Song, C. A New Approach to Deep Desulfurization of Gasoline, Diesel Fuel and Jet Fuel by Selective Adsorption for Ultra-Clean Fuels and for Fuel Cell Applications. Catal. Today (2002), 77, 107 and Ma, X.; Sprague, M.; Sun, L.; Song, C. Deep Desulfurization of Liquid Hydrocarbons by Selective Adsorption for Fuel Cell Applications. Am. Chem. Soc. Div. Pet. Chem. Prepr. (2002), 47, 48. For jet fuel, they obtained a saturation adsorption capacity of 0.015 g of sulfur per cm3 of adsorbent and also showed that breakthrough occurs at about 20 cm3 effluent volume for about 3.2 cm3 of the metal loaded silica gel. For a model diesel fuel, Ma et al. obtained a breakthrough capacity of 1 cm3 per gram of adsorbent. The latter was done for removal of dibenzothiophene and 4,6-dibenzothiophene molecules. Collins et al. also performed fixed-bed adsorption experiments for sulfur removal, but after oxidation of the thiophenic compounds. Collins, F. M.; Lucy, A. R.; Sharp, C. Oxidative Desulphurisation of Oils via Hydrogen Peroxide and Heteropolyanion Catalysis. J. Mol. Catal. A-Chem. (1997), 117, 397. Oxidation was accomplished by using hydrogen peroxide, an acid catalyst and a phase transfer agent. Afterwards, the oxidized sulfurs were removed from the diesel oil using a silica gel. A breakthrough capacity of about 11 cm3 per gram of silica gel was obtained in this case. Another adsorbent that has been studied was ALCOA Selexsorb, which is an activated alumina. In one specific application (Irvine, R. L. Process for Desulfurizing Gasoline and Hydrocarbon Feedstocks. U.S. Pat. No. 5,730,860, 1998) this proprietary material was used in a temperature swing adsorption (TSA) process in order to continuously adsorb hetereoatoms from hydrocarbon mixtures and produce full boiling range FCC gasoline products with a maximum sulfur content of 30 ppmw.
Essentially all industrial adsorption processes are based on van der Waals interactions between the sorbate and the sorbent. Chemical bonds have yet to be exploited. Further, the drawbacks concomitant with HDS appear to render it an inappropriate solution.
Thus, it would be desirable to provide an adsorption process for selectively removing sulfur compounds from liquid fuels at ambient conditions, thereby advantageously leading to a major advance in petroleum refining. It would further be desirable to provide highly selective sorbents for this process, thereby overcoming the drawbacks of current commercial sorbents, which are not desirable for this application. Still further, it would be desirable to provide an adsorption process for selectively removing aromatic compounds from a mixture of aromatic and aliphatic compounds.